Study on AC Flashover Performance for Different Types of Porcelain and Glass Insulators With Non-Uniform Pollution

The ratio <i>T</i>/<i>B</i> of top to bottom surface salt deposit density (SDD) affects the ac pollution flashover performance of disk insulator strings. The ac pollution flashover stress was established for fifteen different combinations of SDD and T/B in a systematic study, making use of seven-unit suspension strings with six different disk profiles in this paper. Then a comparison was made of pollution performance for glass and porcelain disks with the same profile. The observed relation of ac flashover stress <i>EL</i> to SDD and <i>T</i>/<i>B</i> followed an equation of the form <i>EL</i>=<i>c</i>·SDD^-<i>b</i>·(1-<i>A</i>·log(<i>T</i>/<i>B</i>)) . The values of <i>c</i> , <i>b</i> and <i>A</i> were fitted to test results for glass and porcelain disks of identical bottom-rib profile, and to four other bottom-rib and external-rib profiles. A reduction in the ratio <i>T</i>/<i>B</i> from 1/1 to 1/15 gave a median 26% ± 8% increase in flashover strength, corresponding to the calculated increase in overall pollution layer resistance. Extrapolation of results for the seven-unit strings to UHV dimensions suggests that some reduction in leakage distance can be accepted in areas where there is frequent natural washing of the top surfaces of disk insulators.